Dear Mike, what you observed is simple to explain. As you told yourself, freshly spawned eggs are the best caviar of the world. It is quite natural that the Sphaerichthys fed on them; they will do so in nature too, many other co-inhabitants of the Paro’s habitats do if they get them. Normally this is difficult for two reasons. Firstly, the licorice gouramis look for caves, that means small darker rooms with a near bottom and and near ceiling. Since the eggs ar heavier than water they will inevitably sink down after having been produced by the female and inseminated by the male. But many of them will be hold by the bend of the parents’ bodies and especially the anal fin of the female. Nevertheless they will continue to sink down after the torpidity of the parents has ended. But if there is no free or long way to the bottom where they could disappear but a short way of an inch or less, the male and the female are programmed to catch them and bring them to the ceiling of the cave. So, the first thing one should do at any rate is to present small caves (in a small tank of ten or twenty liters and one pair only one is enough) which prevent the eggs from sinking a long distance to the ground. But there is a second mistake you made (if we let apart that in presence of other fish these always will try to feed on the fresh eggs of a spawing pair) and this is that you most surely used the wrong water. So this is the second reason why in nature the enemies will not get many eggs: caught by the parents and brought up to the surface of the cave they will stick.
It was the main discovery of Dr. Walter Foersch in the seventies of last century what the reason for non-sticking is. And therefore we call him the father of Parosphromenus-aquaristics. He executed a long series of experiments that doubtlessly proved water conductivity to be the decisive factor for that problem. Especially the lime-content of the water is resonsible for non-sticking eggs. Mind, that in the home waters of Paros there is no lime at all. You should provide such a water at any rate, otherwise the eggs – if collected and brought to the ceiling of the cave – will not stick. Materials of the cave, the structure of the surface, the colour and other factors are irrelevant. It is – as far as we know today – the lime-free water that ensures the sticking. Cheap papers or sticks for measuring that are useless; they are much too inaccurate. You must use at least fluid measuring kits, better electronic equipment (which is calibrated correctly!). Even slight amounts of lime suffice to prevent sticking. The aim is: no lime at all. There can be some other ingredients, there must be, for no organism can live in pure H2O. But the licorice are near to that. Many breeders use pure water from their reverse-osmosis-system, or pure rainwaters from non-polluted areas, or they mix pure destilled water with a few minerals and humine substances, but no lime. Humine substances are important, too. But the main thing is freeness (or nearly freeness) of lime. Your water, whatever you used, could not have been of this quality. Otherwise the eggs would stick.
Many people seem to think they could produce the right Paro-water by adding industrial additives offered by the aquarium industry. That’s nonsense. You only could produce the right water by removing not by adding. You must remove solved minerals from the water, and that’s the solution for this problem. Only humine substances could and should be added, all others must be removed. The pH is irrelevant for all this. It’s a means for keeping bacteria and other germs at a low level. This is important for the life of the eggs and in the eggs, but not for the sticking or non-sticking.
I once have seen a very fine aquarium of about 200 liters that hosted a nice blackwater community of Parosphromenus, Sphaerichthys and some others. But the owner bred his fish himself in separate small tanks. When there were many of them, he composed that fine bigger community tank. I think, this is the right way to act.